Gnuplot draws 3D figures with lines and / or points, in addition,
ver.3.8 or 4.0 allows you to draw a color-mapped 3D figure by setting
pm3d . The figure is shown on your screen as well as
PostScript or some image formats like PNG/JPG.

You can choose color or gray-scale by the terminal option of
color/monochrome . The gray-scale figure can be drawn
by set palette gray , but the surface grid lines are
still colored in this case. The following two examples are
for the PostScript terminal.

A contour map of gnuplot is shown by lines. The colored contour by pm3d
is something like a colored density distribution, which means,
each piece of colored surface is mapped on the bottom/top plain.
To show this on the bottom, add the at b option
to the set pm3d command.

Since gnuplot cannot draw a 3D-bar graph, we need a little trick
to make it. Suppose we have the following 3D data.

# X Y Z
0.0 0.0 2.0
0.0 1.0 3.5
1.0 0.0 1.0
1.0 1.0 3.0

The first line means Z=2 when 0< X <1, 0< Y <1, and
it corresponds to the area (1) in the next figure. The data file does not
contain maximal values. The ranges of X and Y are implicitly
assumed as 1< Y <2, 1< Y <2, and the Z value is 3.0.
We will make a 3D histgram like this.

Now we expand the data with some programs/tools. See the figure
above at X=0, the Y values are step function changing Y=2.0, 2.0, 3.5,
and 3.5. This step function is expressed as one line in a 3D
space. Same can be consider for X=1 and X=2.